Luc, using that equipment, the sub and speakers could be connected in two ways: first, you can run speaker wire from the amp terminals to the sub high level inputs and run separate speaker wires directly from the amp terminals to the M3 terminals(jamming both into the amp terminals and possibly trimming the ends down in size to help fit); second, you could run a speaker wire from the sub high level input terminals(again, jamming both wires in)out to the M3 terminals. These two connections are electrically identical. Note that subs don't have internal crossovers which might affect speakers connected through the sub, and the speakers will continue to run full-range.

Thanks John, that's what I suspected but it just seems strange that they wouldn't include an output for this on the sub.

Per Alan's "Receiver and Subwoofer crossover set up" in the Articles section of the site the sub does have a crossover to be used in this case, I just wasn't clear on how to output from the sub

If you are running an older two-channel stereo receiver or an integrated amplifier that lacks a subwoofer output jack, connect the subwoofer by running the receiver’s left and right speaker cables to the subwoofer’s high-level or "speaker-level" input connectors. A second set of speaker cables would then run between the subwoofer and your main left and right front speakers. In this scenario, you would adjust the subwoofer crossover control to achieve a smooth blend between your main speaker’s bass output and the subwoofer’s deep bass output.

No, subs actually don't have internal crossovers, even if the term is used. There's only a low-pass filter which affects only the sub, even if a speaker is connected through the sub(the connection is straight-through, and as mentioned above is electrically identical to connecting the speaker directly to the receiver/amplifier terminals). The speakers continue to run full-range, and an attempt to blend their natural roll-off with the sub output is made by adjusting the sub's low-pass filter to match that roll-off.

As to why separate speaker level output terminals aren't included on the sub, they're only a minor convenience since either of the two connections described above can still be made and have the same result.